Various home appliances such as TVs or refrigerators operate by receiving the commercial AC power from the outside. Electronic devices such as laptop computers, mobile phone terminals, or tablet terminals are also operable by a commercial AC power, or batteries built in devices are chargeable by the commercial AC power. A power supply device (AC/DC converter) that AC/DC-converts the commercial AC voltage is incorporated in such home appliances or electronic devices (hereinafter, generally referred to as “electronic devices”). Alternatively, the AC/DC converter may be incorporated in an external power adapter (AC adapter) of the electronic devices.
FIG. 1 is a block diagram illustrating a basic configuration of an AC/DC converter 100r reviewed by the present inventors. The AC/DC converter 100r largely includes a filter 102, a rectifier circuit 104, a smoothing capacitor 106, and an insulation type DC/DC converter 200r. 
A commercial AC voltage VAC is input to the filter 102 via a fuse and an input capacitor (not shown). The filter 102 removes the noise of the commercial AC voltage VAC. The rectifier circuit 104 is a diode bridge circuit that full-wave rectifies the commercial AC voltage VAC. An output voltage of the rectifier circuit 104 is smoothed by the smoothing capacitor 106 and converted into a DC voltage VIN.
The insulation type DC/DC converter 200r receives the DC voltage VIN by an input terminal P1, steps down the same, and supplies an output voltage VOUT stabilized to a target value to a load (not shown) connected to an output terminal P2.
The DC/DC converter 200r includes a primary side controller 202, a photocoupler 204, a feedback circuit 206, an output circuit 210, a synchronous rectification controller 300r, and other circuit components. The output circuit 210 includes a transformer T1, a diode D1, an output capacitor C1, a switching transistor M1, and a synchronous rectifying transistor M2. The topology of the output circuit 210 is the same as that of a general synchronous rectification type flyback converter, and thus, a description thereof will be omitted.
As the switching transistor M1 connected to a primary winding W1 of the transformer T1 is switched, the input voltage VIN is stepped down to generate the output voltage VOUT. Further, the primary side controller 202 adjusts a duty ratio of switching of the switching transistor M1.
The output voltage VOUT of the DC/DC converter 200r is divided by resistors R1 and R2. The feedback circuit 206 includes, for example, a shunt regulator or an error amplifier; and amplifies an error between a divided voltage (voltage detection signal) Vs and a predetermined reference voltage VREF (not shown), generates an error current IERR corresponding to the error, and draws (sink) it from a light emitting device (light emitting diode) on an input side of the photocoupler 204.
A feedback current IFB corresponding to the error current IERR of the secondary side flows through a light receiving device (photo transistor) on the output side of the photocoupler 204. The feedback current IFB is smoothed by a resistor and a capacitor and input to a feedback (FB) terminal of the primary side controller 202. The primary side controller 202 adjusts a duty ratio of the switching transistor M1 based on a voltage (feedback voltage) VFB of the FB terminal.
The synchronous rectification controller 300r switches the synchronous rectifying transistor M2 in synchronization with the switching of the switching transistor M1. The synchronous rectification controller 300r includes a pulse generator 304 and a driver 306. The pulse generator 304 generates a pulse signal S1 synchronized with the switching of the switching transistor M1. For example, when the switching transistor M1 is turned off, the pulse generator 304 sets the pulse signal S1 to a first state (e.g., a high level) indicating ON of the synchronous rectifying transistor M2. Further, when the secondary current IS flowing through the secondary winding W2 becomes substantially zero during an ON period of the synchronous rectifying transistor M2, the synchronous rectification controller 300r sets the pulse signal S1 to a second state (a low level) indicating OFF of the synchronous rectifying transistor M2.
Since a voltage across the secondary winding W2 is −VIN×NS/NP during the ON period of the switching transistor M1, a drain voltage VD (i.e., a drain-source voltage VDS) of the synchronous rectifying transistor M2 becomes VD=VOUT+VIN×NS/NP. NP and NS are the numbers of turns of the primary winding W1 and the secondary winding W2, respectively.
When the switching transistor M1 is turned off, since the secondary current IS flows from the source of the synchronous rectifying transistor M2 to the drain thereof, the drain-source voltage becomes a negative voltage. In a continuous mode, as the switching transistor M1 is turned on, the secondary current IS becomes zero and the drain voltage again jumps to VD=VOUT+VIN×NS/NP. In a discontinuous mode, when the secondary current IS decreases as the energy stored in the transformer T1 decreases in the ON state of the synchronous rectifying transistor M2, an absolute value of the drain-source voltage VDS decreases, and when the secondary current IS eventually becomes substantially zero, the drain-source voltage VDS also becomes substantially zero and the drain voltage VD is ringing.
Using these properties, the pulse generator 304 generates the pulse signal S1 based on the drain voltage (the drain-source voltage) of the synchronous rectifying transistor M2.
The driver 306 switches the synchronous rectifying transistor M2 depending on the pulse signal S1. The above is the overall configuration of the AC/DC converter 100r. 
It is assumed that the DC/DC converter 200r of FIG. 1 fails in a state in which the synchronous rectifying transistor M2 is turned off. Then, the secondary side circuit of the DC/DC converter 200r continues to operate as a diode rectifier circuit by a body diode of the synchronous rectifying transistor M2. Since a voltage drop of 0.7V is always generated in the body diode of the synchronous rectifying transistor M2, the synchronous rectifying transistor M2 abnormally generates heat.
FIG. 2 is a circuit diagram of a secondary side circuit 220r of the DC/DC converter 200r reviewed by the present inventors. The secondary side circuit 220r should not be recognized as a known technique.
The secondary side circuit 220r includes a thermistor 222 and a protection circuit 230, in addition to the secondary side circuit of FIG. 1. The thermistor 222 is installed to measure a package temperature of the synchronous rectifying transistor M2. One end of the thermistor 222 is connected to a connection line of the secondary winding W2 and the output capacitor C1, so that the DC voltage VOUT is applied as a DC bias voltage. A voltage drop corresponding to a temperature of the synchronous rectifying transistor M2 is generated in the thermistor 222. The protection circuit 230 detects an overheated state of the synchronous rectifying transistor M2 based on a voltage VNTC generated at the other end of the thermistor 222. Upon detecting the overheated state, the protection circuit 230 drives the light emitting diode of the photocoupler 204 with a large current. Thus, the feedback voltage VFB of the primary side becomes zero and the duty ratio of the switching transistor M1 becomes zero, namely the switching is stopped.
FIGS. 3A and 3B are diagrams illustrating layouts of the thermistor 222 and a package 224 of the synchronous rectifying transistor M2. In FIG. 3A, the thermistor 222 is mounted on a rear surface of a printed board 226 in which the package 224 of the synchronous rectifying transistor M2 is mounted on its surface. In FIG. 3A, since the printed board 226 has thermal resistance, it is difficult to say that the thermistor 222 can accurately measure even the temperature of the package 224, let alone the temperature of a field effect transistor (FET) in the package 224.
In FIG. 3B, a lead type thermistor 222 is densely mounted on a surface of the package 224 of the synchronous rectifying transistor M2. According to this layout, the temperature of the package 224 can be accurately measured, as compared with that of FIG. 3A, but it is difficult to accurately measure the temperature of the FET in the package 224 due to the thermal resistance of a mold resin of the package.